A new rhynchonellid genus, Momarhynchus, with type species M. indigirkaensis n. sp., is described from the lower Famennian of eastern Yakutia, Russia, North-East Asia. The new genus and the new species allow us to better define the so-called Leiorhynchus ursus Zone in the area concerned. This zone, also referred to as the Zigania ursa Zone, is recognized over a wide territory of the former USSR, and is included in some Regional Unified Stratigraphical Schemes; its understanding is unfortunately hampered by technicalities.